Double sided wire grid polarizer

ABSTRACT

A wire grid polarizer ( 100 ) for polarizing an incident light beam ( 130 ), comprising a substrate ( 505 ) having a first surface ( 410 ) and a second surface ( 510 ); and a first array of parallel, elongated wires disposed on the first surface ( 410 ). Each of the wires are spaced apart at a grid period less than a wavelength of the incident light; and a second array of parallel, elongated wires disposed on said second surface ( 420 ) where the second array of wires are oriented parallel to the first array of wires.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 09/977,544, filed Oct.15, 2001, now U.S. Pat. No. 6,714,350.

FIELD OF THE INVENTION

The present invention generally relates to wire grid polarizers andtheir use in a modulation optical system. The present invention relatesin particular to double sided wire grid polarizers and beamsplitters forthe visible spectrum, and the use of these double sided wire gridpolarizers within a modulation optical system.

BACKGROUND OF THE INVENTION

The use of an array of parallel conducting wires to polarize radio wavesdates back more than 110 years. Wire grids, generally in the form of anarray of thin parallel conductors supported by a transparent substrate,have also been used as polarizers for the infrared portion of theelectromagnetic spectrum.

The key factor that determines the performance of a wire grid polarizeris the relationship between the center-to-center spacing, sometimesreferred to as period or pitch, of the parallel grid elements and thewavelength of the incident light. If the grid spacing or period is longcompared to the wavelength, the grid functions as a diffraction grating,rather than as a polarizer, and diffracts both polarizations, notnecessarily with equal efficiency, according to well-known principles.However, when the grid spacing (p) is much shorter than the wavelength,the grid functions as a polarizer that reflects electromagneticradiation polarized parallel (“s” polarization) to the grid, andtransmits radiation of the orthogonal polarization (“p” polarization).The transition region, where the grid period is in the range of roughlyone-half of the wavelength to twice the wavelength, is characterized byabrupt changes in the transmission and reflection characteristics of thegrid. In particular, an abrupt increase in reflectivity, andcorresponding decrease in transmission, for light polarized orthogonalto the grid elements will occur at one or more specific wavelengths atany given angle of incidence. These effects were first reported by Woodin 1902, and are often referred to as “Wood's Anomalies.” Subsequently,in 1907, Rayleigh analyzed Wood's data and had the insight that theanomalies occur at combinations of wavelength and angle where a higherdiffraction order emerges. Rayleigh developed following equation topredict the location of the anomalies, which are also commonly referredto in the literature as “Rayleigh Resonances.”λ=ε(n+/−sin θ)/k  (1)wherein epsilon (ε) is the grating period; n is the refractive index ofthe medium surrounding the grating; k is an integer corresponding to theorder of the diffracted term that is emerging; and lambda and theta arethe wavelength and incidence angel (both measured in air) where theresonance occurs.

For gratings formed on one side of a dielectric substrate, n in theabove equation may be equal to either 1, or to the refractive index ofthe substrate material. Note that the longest wavelength at which aresonance occurs is given by the following formula:λ=ε(n+sin θ)  (2)where n is set to be the refractive index of the substrate.

The effect of the angular dependence is to shift the transmission regionto larger wavelengths as the angle increases. This is important when thepolarizer is intended for use as a polarizing beamsplitter or polarizingturning mirror.

In general, a wire grid polarizer will reflect light with its electricfield vector parallel (“s” polarization) to the wires of the grid, andtransmit light with its electric field vector perpendicular (“p”polarization) to the wires of the grid, but the plane of incidence mayor may not be perpendicular to the wires of the grid as discussed here.Ideally, the wire grid polarizer will function as a perfect mirror forone polarization of light, such as the S polarized light, and will beperfectly transparent for the other polarization, such as the Ppolarized light. In practice, however, even the most reflective metalsused as mirrors absorb some fraction of the incident light and reflectonly 90% to 95%, and plain glass does not transmit 100% of the incidentlight due to surface reflections. The performance of wire gridpolarizers, and indeed other polarization devices, is mostlycharacterized by the contrast ratio, or extinction ratio, as measuredover the range of wavelengths and incidence angles of interest. For awire grid polarizer or polarization beamsplitter, the contrast ratiosfor the transmitted beam (Tp/Ts) and the reflected beam (Rs/Rp) may bothbe of interest.

Historically, wire grid polarizers were developed for use in theinfrared, but were unavailable for visible wavelengths. Primarily, thisis because processing technologies were incapable of producing smallenough sub-wavelength structures for effective operation in the visiblespectrum. Nominally, the grid spacing or pitch (p) should be less than˜λ/5 for effective operation (for p˜0.10-0.13 μm for visiblewavelengths), while even finer pitch structures (p˜λ/10 for example) canprovide further improvements to device contrast. However, with recentadvances in processing technologies, including 0.13 μm extreme UVphotolithography and interference lithography, visible wavelength wiregrid structures have become feasible. Although there are severalexamples of visible wavelength wire grid polarizers devices known in theart, these devices do not provide the very high extinction ratios(>1,000:1) across broadband visible spectra needed for demandingapplications, such as for digital cinema projection.

An interesting wire grid polarizer is described by Garvin et al. in U.S.Pat. No. 4,289,381, in which two or more wire grids residing on one sideof a single substrate are separated by a thin dielectric interlayer.Each of the wire grids are deposited separately, and the wires are thickenough (100-1000 nm) to function as a polarizer without significantlight leakage through the metal wires. As the dielectric interlayer isthick enough to avoid resonance, the wire grids effectively multiply,such that while any single wire grid may only provide 500:1 polarizationcontrast, in combination a pair or grids may theoretically provide250,000:1. This device is described relative to usage in the infraredspectrum (2-100 μm), although presumably the concepts are extendable tovisible wavelengths. However, as this device employs two or more wiregrids in a series, the additional contrast ratio is exchanged forreduced transmission efficiency and angular acceptance. Furthermore, thedevice is not designed for high quality extinction for the reflectedbeam, which places some limits on its value as a polarizationbeamsplitter.

A wire grid polarization beamsplitter for the visible wavelength rangeis described by Hegg et al. in U.S. Pat. No. 5,383,053, in which themetal wires (with pitch p<<λ and ˜150 nm features) are deposited on topof metal grid lines, each of which are deposited onto glass or plasticsubstrate. While this device is designed to cover much of the visiblespectrum (0.45-0.65 μm), the anticipated polarization performance israther modest, delivering an overall contrast ratio of only 6.3:1.

Tamada et al., in U.S. Pat. No. 5,748,368, describes a wire gridpolarizer for the near infrared spectrum (0.8-0.95 μm) in which thestructure of the wires is shaped in order to enhance performance. Inthis case, operation in the near infrared spectrum is achieved with awire structure with a long grid spacing (λ/2<p<λ) rather than thenominal small grid spacing (p˜λ/5) by exploiting one of the resonancesin the transition region between the wire grid polarizer and thediffraction grating. The wires, each ˜140 nm thick, are deposited on aglass substrate in an assembly with wedge plates. In particular, thedevice uses a combination of trapezoidal wire shaping, index matchingbetween the substrate and a wedge plate, and incidence angle adjustmentto tune the device operation to hit a resonance band. While this deviceprovides reasonable extinction of ˜35:1, which would be useful for manyapplications, this contrast is inadequate for applications, such asdigital cinema, which require higher performance. Furthermore, thisdevice only operates properly within narrow wavelength bands (˜25 nm)and the device is rather angularly sensitive (a 2° shift in incidenceangle shifts the resonance band by ˜30 nm). These considerations alsomake the device unsuitable for broadband wavelength applications inwhich the wire grid device must operate in “fast” optical system (suchas F/2.5).

Most recently, U.S. Pat. No. 6,108,131 (Hansen et al.), U.S. Pat. No.6,122,103 (Perkins et al.) and U.S. Pat. No. 6,243,199 (Hansen et al.),all assigned to Moxtek Inc., or Orem, Utah, describe wire grid polarizerdevices designed for the visible spectrum. U.S. Pat. No. 6,108,131describes a straightforward wire grid polarizer designed to operate inthe visible region of the spectrum. The wire grid nominally consists ofa series of individual wires fabricated directly on a substrate with a˜0.13 μm gridline spacing (p˜λ/5), wire nominal width of 0.052-0.078 μmwide (w), and wire thickness (t) greater than 0.02 μm. By using wires of˜0.13 μm grid spacing or pitch, this device has the required sub-visiblewavelength structure to allow it to generally operate above the longwavelength resonance band and in the wire grid region. U.S. Pat. No.6,122,103 proposes a variety of improvements to the basic wire gridstructure directed to broadening the wavelength spectrum and improvingthe efficiency and contrast across the wavelength spectrum of usewithout requiring finer pitch structures (such as ˜λ/10). Basically, avariety of techniques are employed to reduce the effective refractiveindex (n) in the medium surrounding the wire grid, in order to shift thelongest wavelength resonance band to shorter wavelengths (see equations(1) and (2)). This is accomplished most simply by coating the glasssubstrate with a dielectric layer which functions as ananti-reflectional (AR) coating, and then fabricating the wire grid ontothis intermediate dielectric layer. The intermediate dielectric layereffectively reduces the refractive index experienced by the light at thewire grid, thereby shifting the longest wavelength resonance shorter.U.S. Pat. No. 6,122,103 also describes alternate designs where theeffective index is reduced by forming grooves in the spaces between thewires, such that the grooves extend into the substrate itself, and/orinto the intermediate dielectric layer which is deposited on thesubstrate. As a result of these design improvements, the low wavelengthband edge shifts ˜50-75 nm lower, allowing coverage of the entirevisible spectrum. Furthermore, the average efficiency is improved by ˜5%across the visible spectrum over the basic prior art wire gridpolarizer. By comparison, U.S. Pat. No. 6,243,199 patent describes howto optimize wire pitch (p), wire thickness(t), wire width (w), and wireprofile in order to control throughput and contrast for visible wiregrid polarizers.

Although these new visible spectrum wire grid polarizers andpolarization beam splitters provide enhanced contrast compared to thestandard technologies (for example the MacNielle prism, U.S. Pat. No.2,403,731), some applications require complex polarization opticsarrangements to obtain the desired contrast levels, which can utilizemore than one wire grid device. For example, to attain the 1,000:1⁺system contrast required of a digital cinema projection system, amodulation optical system that includes two wire grid polarizers and onewire grid polarization beam splitter may be utilized. An electronicprojection system of this type may use high modulation contrast liquidcrystal displays (LCDs) and high power (up to 6 kW) xenon arc lamps todeliver 1,000:1 system CR and 10,000 screen lumens. The design of such asystem is complicated by the effects of thermal loading on the opticalcomponents, mechanical fixtures, and electrical circuitry. Inparticular, thermal loading of the polarization optics (LCDs, polarizersand the polarization beam splitter) can cause stress birefringence,thereby reducing the screen contrast. Furthermore, thermal loading ofthe polarization beam splitter, and in particular a sheet polarizingbeam splitter (such as a wire grid device) can cause surface profiledeformations which could effect the wavefront quality of the imagebearing light beam. This can in turn translate into a degradation of thescreen resolution of the projected image.

Therefore it is desirable to provide enhanced wire grid polarizationbeam splitters which have reduced sensitivity to thermal stress, andwhich can therefore improve the optical performance of the modulationoptical system in which they reside.

Furthermore, it is desirable to provide an enhanced wire gridpolarization beam splitter which provides enhanced polarizationperformance over those devices known in the prior art. In particular,while the devices described in U.S. Pat. Nos. 6,108,131, 6,122,103 and6,243,199 are definite improvements over the prior art, there are yetfurther opportunities for performance improvements for both wire gridpolarizers and polarization beamsplitters. For example, in electronicprojection systems utilizing reflective LCD modulators, the imagebearing light interacts with the PBS both in reflection andtransmission. Thus, having high polarization extinction for both thereflected and transmitted beams is valuable. As the commerciallyavailable wire grid polarizers from Moxtek provide only ˜20:1 contrastfor the reflected channel, rather than 100:1 or even 2,000:1, the designof optical systems using these devices must be configured, with somedifficulty, to compensate for the low reflective contrast. Additionally,the performance of these devices varies considerably across the visiblespectrum, with the polarization beamsplitter providing contrast ratiosfor the transmitted beam varying from ˜300:1 to ˜1200:1 from blue tored, while the reflected beam contrast ratios vary from 10:1 to 30:1.Thus, there are opportunities to provide polarization contrastperformance in the blue portion of the visible spectrum in particular,as well as more uniform extinction across the visible. There are alsoopportunities to improve the combined polarization contrast and lightefficiency for the transmitted p-polarization light beyond the levelsprovided by prior art wire grid devices.

Thus, there exists a need for an improved wire grid polarizationbeamsplitter, particularly for use in visible light systems requiringbroad wavelength bandwidth and high contrast (target of 1,000:1 orgreater). In addition, there exists a need for such an improved wiregrid polarization beam splitter for use at incidence angles of about 45degrees.

SUMMARY OF THE INVENTION

A wire grid polarizer for polarizing an incident light beam, comprisinga substrate having a first surface and a second surface, and a firstarray of parallel, elongated conducting wires disposed on said firstsurface. Each of the wires are spaced apart at a grid period less than awavelength of the incident light. A second array of parallel, elongatedconducting wires disposed on the second surface where the second arrayof wires are oriented parallel to the first array of wires. The wires ofthe second array are likewise spaced apart at a grid period less than awavelength of the incident light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a prior art wire grid polarizer.

FIGS. 2 a and 2 b are plots illustrating the relative performance orprior art wire grid polarizers and polarization beamsplitters designedto operate within the visible spectrum.

FIGS. 3 a and 3 b are plots of transmitted, reflected, and overallpolarization contrast ratios vs. wavelength in the visible spectrum fora wire grid polarization beamsplitter of a type described in the priorart.

FIG. 4 is a cross sectional view showing a modulation optical systemwhich includes a wire grid polarization beamsplitter.

FIG. 5 is a detailed cross sectional view of the double sided wire gridpolarizer of the present invention.

FIGS. 6 a and 6 b are cross sectional views showing modulation opticalsystems utilizing the double sided wire grid polarization beamsplitterof the present invention.

FIGS. 7 a-7 g are graphic plots showing wire grid performancecharacteristics of related to design of a double sided wire gridpolarizer of the present invention.

FIGS. 8 a-8 d are cross sectional views of various wire structures whichcould be employed in the wire grid polarizer of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made to the drawings in which the various elementsof the present invention will be given numerical designations and inwhich the invention will be discussed so as to enable one skilled in theart to make and use the invention.

FIG. 1 illustrates a basic prior art wire grid polarizer and definesterms that will be used in a series of illustrative examples of theprior art and the present invention. The wire grid polarizer 100 iscomprised of a multiplicity of parallel conductive electrodes 110supported by a dielectric substrate 120. This device is characterized bythe grating spacing or pitch or period of the conductors, designated(p); the width of the individual conductors, designated (w); the dutycycle (w/p) and the thickness of the conductors, designated (t).Nominally, a wire grid polarizer uses sub-wavelength structures, suchthat the pitch (p), conductor or wire width (w), and the conductor orwire thickness (t) are all less than the wavelength of incident light(λ). As the conductive electrodes are required to be highly electricallyconductive, these wires are nominally metallic, and are for example,made of aluminum. A beam of light 130 produced by a light source 132 isincident on the polarizer at an angle θ from normal, with the plane ofincidence orthogonal to the conductive elements. The wire grid polarizer100 divides this beam into specular non-diffracted outgoing light beams;reflected light beam 140 and transmitted light beam 150. The definitionsfor S and P polarization used are that S polarized light is light withits polarization vector parallel to the conductive elements, while Ppolarized light has its polarization vector orthogonal to the conductiveelements.

Referring to FIG. 2 a there is shown, for wavelengths within the visiblespectrum, the transmission efficiency curve 200 and the transmitted “p”polarization contrast ratio curve 205 for a commercially available wiregrid polarization beamsplitter from Moxtek Inc., of Orem, Utah. Thisdevice is similar to the basic wire grid polarization beamsplitterdescribed in U.S. Pat. No. 6,108,131, which has ˜130 nm pitch (p˜λ/5)aluminum wires (parallel conductive electrodes 110) made with a 40-60%duty cycle (52-78 nm wire width (w)) deposited on a dielectric substrate120. The solid metal wires are nominally 100-150 nm thick, whichprovides sufficient metal thickness to exceed the skin depth (δ) of themetal layer in the visible spectrum, thereby enhancing the devicecontrast. (Skin depth is an estimate of the metal layer thickness atwhich light tunneling through the layer is minimal.) This data isrepresentative for this device for a modest NA (numerical aperture)light beam, incident on the wire grid polarization beam splitter 100 atan angle of incidence (θ) of 45°. As this device divides the incidentbeam of light 130 into two outgoing polarized beams (140 and 150), thattravel paths spatially distinguishable from the incoming light path,this device is considered to be a polarizing beamsplitter. Thetransmitted contrast ratio curve 205 measures the average contrast ofthe transmitted “p” polarized light, relative to the transmitted “s”polarized light (Tp/Ts), where the “s” polarized light is undesirableleakage. Likewise, the reflected contrast ratio curve 210 measures theaverage contrast of the reflected “s” polarized light relative to the“p” polarized light (Rs/Rp). As measured across the visible spectrumfrom blue to red, the transmitted contrast ranges from ˜300-1200:1 whilethe reflected contrast ranges from ˜10-40:1.

Referring to FIG. 2 b, there is shown for wavelengths within the visiblespectrum, the average performance for a commercially available wire gridpolarizer from Moxtek which is designed for use with normally incident(θ=0°) beam of light 130. In particular, the transmission efficiencycurve 220 and the transmitted contrast ratio curve 225, are provided(for “p” polarized light). In this case, the measured transmittedcontrast exceeds 200:1 for much of the green and red spectra. Theperformance of both of these devices, relative to contrast, wavelengthresponse, angular response, transmission, and robustness is very good ascompared to the historically available alternatives, and is satisfactoryfor many applications. It should be noted that the wire grid designs forthe wire grid polarizer (used at normal incidence) and the wire gridpolarization beam splitter (used at non-normal incidence) may bedifferent and optimized for their intended uses.

The preferred spatial relationships of these polarizers, as used in amodulation optical system 300, are illustrated in FIG. 4. Modulationoptical system 300, which is a portion of an electronic projectionsystem, comprises an incoming illumination light beam 320, focusedthrough prepolarizer 330, wire grid polarization beamsplitter 340,optional compensator 360, and onto spatial light modulator 310 (the LCD)by a condensor 325. A modulated, image-bearing light beam is reflectedfrom the surface of spatial light modulator 310, transmitted throughcompensator 360, reflected off the near surface of wire gridpolarization beamsplitter 340, and subsequently transmitted through asecond compensator 365 (optional), a polarization analyzer 370, andrecombination prism 380. Recombination prism 380 is typically anx-prism, although crossed dichroic plates could be used (aside from theresulting tilted plate optical aberrations). The modulation contrastprovided by this system is not only impacted by the performance of theindividual polarization components (spatial light modulator 310,prepolarizer 330, wire grid polarization beamsplitter 340, compensator360, and polarization analyzer 370), but also by performance variabilitywithin these components caused by thermal stress induced birefringencechanges under high heat (light) loads. Likewise, thermal loading cancause surface deformations of these various optics, thereby deformingthe wavefronts of the transiting image bearing light, and thus affectingthe projected on screen image quality (resolution). It is howeverpossible to provide an enhanced wire grid polarization beam splitter340, which will be desensitized to thermally induced changes and alsoprovide optimized performance (relative to contrast and lightefficiency), thereby improving the overall performance of modulationoptical system 300.

These opportunities can be better understood by first explaining theperformance deficiencies of the existing wire grid devices (see FIG. 2 afor the wire grid polarization beamsplitter and FIG. 2 b for the wiregrid polarizer). In particular, the contrast ratio of the reflected “s”polarized beam is rather low, as measured by the reflected contrastratio curve 210, for the wire grid polarizing beamsplitter. Polarizationcontrast is only ˜10:1 in the blue spectrum (at 450 nm), and even in thered (650 nm), it has risen only to ˜40:1. In the case of modulationoptical system 300 of FIG. 4 applications, where the contrast isdetermined by both reflective and transmissive interactions with thewire grid polarization beam splitter 340, this performance isinsufficient by itself. Additionally, while this prior art wire gridpolarization beamsplitter provides contrast ˜1200:1 in the red, thepolarization varies considerably with wavelength, and falls to ˜400:1 inthe low blue (see again transmitted contrast ratio curve 205 of FIG. 2a).

This assessment of the performance of a prior art wire grid polarizationbeamsplitter (as described in U.S. Pat. No. 6,108,131) is betterunderstood by considering the theoretically calculated reflected andtransmitted polarization contrast ratios. This analysis was modeledusing the Gsolver grating analysis software tool, which allowssub-wavelength structures to be thoroughly modeled using rigorouscoupled wave analysis (RCWA). Gsolver is commercially available fromGrating Solver Development Company, P.O. Box 353, Allen, Tex.

The wire grid device was modeled as a series of parallel elongated wiresformed directly on the transparent glass substrate. The analysis assumesan aluminum wire grid with period p=0.13 μm, conductor width w=0.052 μm(40% duty cycle), conductor thickness t=0.182 μm, and substraterefractive index (n) for Coming 1737f glass. The data used in themodeling for the optical properties of the metals can be taken from theHandbook of Optical Constants of Solids, Part I, Edward D. Palik, Ed.,1985, pp. 369-406, for example. For simplicity, this analysis onlyconsiders a collimated beam incident on the wire grid polarizationbeamsplitter at an angle θ=45°.

FIG. 3 a provides the collimated transmitted beam contrast 250 (Tp/Ts)and the collimated reflected beam contrast 255 (Rs/Rp). The calculatedtransmitted beam contrast 250 ranges from 10⁴-10⁵:1 across the visiblespectrum, which is much greater than the ˜1,000:1 levels reported forthe actual device, as shown in FIG. 2 a. However, plot 250 of FIG. 2 arepresents the angle averaged performance of an actual device, whileplot 250 of FIG. 3 a represents the theoretical performance of acollimated beam through a perfect device with slightly thicker wires.FIG. 3 a also shows the theoretical reflected beam contrast 255 asmodeled for this prior art type wire grid devices. The calculatedtheoretical reflected beam contrast ranges from ˜10:1 to ˜100:1 over thevisible spectrum, and is only marginally better than the reflected beamcontrast 255 given in FIG. 2 a for an actual device. FIG. 3 b shows aplot of the theoretical overall contrast ratio 275, where the overallcontrast Cw of the wire grid polarization beamsplitter device isapproximated as:Cw=1/((1/C _(T))+(1/C _(R)))  (3).The overall contrast Cw, which combines the contrast C_(T) of thetransmitted light beam 150 (“p” polarization) with the contrast C_(R) ofthe reflected light beam 140 (“s” polarization), can be seen to bemostly determined by the lowest contrast ratio, which is the contrastfor the reflected light beam. Thus, the overall contrast of the priorart type device per U.S. Pat. No. 6,108,131 is limited by the “s”polarization reflected beam, and is only ˜10:1 to ˜100:1 within thevisible spectrum, with the lowest performance for blue wavelengths.

Considering again FIG. 4, the preferred relationships of the wire griddevices within modulation optical system 300 will be discussed ingreater detail. Modulation optical system 300, which is described inU.S. Pat. No. 6,585,378 (Kurtz et al), should be understood to generallybe a portion of some larger electronic projection system, which includesa light source and power supply, drive electronics, and screen, althoughthis modulation optical system may be used for other applications, suchas image printing.

A full color projection system would employ one modulation opticalsystem 300 per color (red, green, and blue), with the color beamsreassembled through the recombination prism 380. The incomingillumination light beam 320 is focused through prepolarizer 330, wiregrid polarization beamsplitter 340, optional compensator 360, and ontospatial light modulator 310 (the LCD) by a condensor 325. Spatial lightmodulator 310 is packaged within spatial light modulator assembly 315,which includes mounting features, a cover glass, and heat sink (all notshown). Condensor 325, which will likely comprise several lens elements,is part of a more extensive illumination system (not shown) whichtransforms the light from the lamp source into a rectangularly shapedregion of nominally uniform light which nominally fills the active areaof spatial light modulator 310. The modulated, image-bearing light beamreflected from the surface of spatial light modulator 310, istransmitted through compensator 360, is then reflected off the nearsurface of wire grid polarization beamsplitter 340, and is nexttransmitted through a second compensator 365 (optional), a polarizationanalyzer 370, and recombination prism 380. In a modulation opticalsystem 300 utilizing a prior art wire grid polarization beamsplitter,the wire grid polarization beamsplitter 340 consists of a dielectricsubstrate 345 with sub-wavelength wires 350 located on one surface (thescale of the wires is greatly exaggerated in FIG. 4). Thereafter, theimage bearing light is projected down optical axis 375 onto a distantscreen (not shown) by projection lens system 385 (only partially shown).Wire grid polarization beamsplitter 340 is disposed for reflection intoprojection lens system 385, thereby avoiding the well known astigmatismand coma aberrations induced by transmission through a tilted plate.

Compensator 360 is nominally a waveplate which provides a small amountof retardance needed to compensate for geometrical imperfections andbirefringence effects which originate at the surface of spatial lightmodulator 310. Although a compensator 360 will generally be used, someapplications allowing high f# optics may not require it. Compensator365, which is optional, provides contrast enhancements for polarizationresponse errors from other components in the system. For example, in athree color projection system, compensator 365 could be a color tunedwaveplate provided in any given channel to optimize the performancethrough the recombination prism 380. These compensators are used toboost the effective performance of the LCD and wire grid devices, sothat the system contrast levels are met or exceeded.

The construction of modulation optical system 300, as used in a digitalcinema application, is defined both by the system specifications and thelimitations of the available wire grid polarizing devices. In particulardigital cinema requires the electronic projector to provide high framesequential contrast (1,000:1 or better). To accomplish this, thepolarization optical components, excluding spatial light modulator 310(the LCD) of modulation optical system 300 must provide an overallsystem contrast (Cs) of ˜2,000:1. The actual target contrast for thepolarization optics does depend on the performance of the LCDs. Thus, iffor example, the LCDs provide only ˜1500:1 contrast, then thepolarization optics must provide 3,000:1 contrast. For example, an LCDwith vertically aligned molecules is preferred for uses as spatial lightmodulator 310 due to its high innate contrast. Typically, the contrastperformance of both the LCDs and the polarization optics decreases withincreasing numerical aperture of the incident beam. Unfortunately, withtoday's technologies (see FIG. 3 b and overall contrast ratio curve 275)it is not sufficient to use just a single wire grid polarizationbeamsplitter 340 by itself in order to meet the 2,000:1 target contrastfor the polarization optics. For this reason, prepolarizer 330 andpolarization analyzer 370 are both provided as polarization supportcomponents within modulation optical system 300.

As an example, in green light at 550 nm, wire grid prepolarizer 330 hasan angle averaged polarization contrast ratio of ˜250:1. When used incombination, wire grid polarization beamsplitter 340 and wire gridprepolarizer 330 produce a contrast ratio of ˜25:1, which falls wayshort of the system requirements. Thus, the prepolarization performanceof overall modulation optical system 300 is also supported with theaddition of polarization analyzer 370 which is preferably a wire gridpolarizer, and is nominally assumed to be identical to wire gridpolarizer 330. Polarization analyzer 370 removes the leakage of lightthat is of other than the preferred polarization state. Thus, theoverall system contrast Cs for green light, directed through modulationoptical system 300 in its entirety, is boosted to ˜2,900:1, which meetsspecification. Performance does vary considerably across the visiblespectrum, with the same combination of wire grid polarizing devicesproviding ˜3,400:1 contrast in the red spectrum, but only ˜900:1contrast in the blue. Certainly, this performance variation could bereduced with the use of color band tuned devices, if they wereavailable.

The polarization performance of modulation optical system 300 isdetermined by the performance of the wire grid devices in both obviousand non-obvious ways. The overall contrast (Cs) for modulation opticalsystem 300 (ignoring the LCD contribution) can be approximated by:1/Cs=1/(C _(T1) *C _(T2))+1/(C _(R2) *C _(T3))  (4),where C_(T1) is the transmitted contrast of the pre-polarizer 330,C_(T2) and C_(R2) are transmitted and reflected contrast ratios for thewire grid polarization beamsplitter 340, and C_(T3) is the transmittedcontrast for the wire grid polarization analyzer 370. In this system,the overall contrast is largely determined by the low reflected contrastratio C_(R2) for the wire grid polarization beamsplitter 340. Theanalyzer contrast C_(T3) needs to be quite high to compensate for thelow C_(R2) values (˜30:1). Whereas the transmitted contrasts C_(T1) andC_(T2) don't need to be particularly high, provided the respectivecontrast values are reasonably uniform over the spectrum. Potentiallythese wire grid contrasts (C_(T1) and C_(T2)) could be moderated (to˜100:1) each respectively, in order to boost the wire grid transmissionsthrough each of these surfaces. Alternately, if the transmitted contrastC_(T2) was boosted sufficiently (to ˜4,000:1⁺), then pre-polarizer 330could be eliminated. However, the range of wire grid devices that arepresently commercially available are insufficient to enable suchoptimizations of modulation optical system 300. Moreover, even if suchdevices were available, merely replacing the existing wire grid devicesin modulation optical system 300 with better devices that arerefinements of prior art technologies, would not necessarily addressother problems associated with the use of these devices. In summaryequation (4) demonstrates that the manner of combination of the variouspolarization devices can greatly effect the resultant overall contrast.It has also been shown that modulation optical system 300 is bestconstructed with wire grid polarization beamsplitter 340 oriented withthe surface with the sub-wavelength wires 350 facing towards the spatiallight modulator 310, rather than towards the illumination optics(condenser 325) and light source. While the overall contrast (Cs) is˜2,900:1 when this orientation is used, the net contrast dropsprecipitously to ˜250:1 when the alternate orientation (wires on thesurface towards the light source) is used.

Notably, this preferred orientation, with the sub-wavelength wiresoriented towards the modulator, also helps to reduce the impact ofthermal loading on the performance of modulation optical system 300. Itis known that conventional electronic projection systems, using standardglass polarization prisms (MacNeille type), can suffer thermally inducedstress birefringence effects which impact the image contrast. In thesesystems, the localized polarization states of the image bearing lightare altered by thermally induced stress birefringence in the prismswhich was caused by the heat from the high intensity illumination.Modulation optical system 300 of FIG. 4 reduces this problem by havingthe image bearing beam reflect off wire grid polarization beamsplitter340, rather than be transmitted through it. With this configuration,light is transmitted through wire grid polarization beamsplitter 340 onetime only, while the second interaction with the beamsplitter is only asurface interaction. Thermal stress birefringence and the resultingcontrast loss is further reduced because wire grid polarizationbeamsplitter 340 simply uses less glass (˜1.5 mm thick substrates) thandoes the conventional prism (30-50 mm thickness). Nonetheless, contrastloss from stress birefringence with the conventional wire gridpolarization beamsplitters may still occur. Furthermore, a modulationoptical system, such as the one of FIG. 4, with the image bearing lightreflected off of the polarization beam splitter, is more sensitive towavefront errors from thermally induced substrate deformations, than isthe comparable system with the image bearing light transmitted throughthe polarization beam splitter.

From the prior discussion, it can be seen that prior art wire gridpolarization beamsplitters, while superior to more conventionaltechnologies, have various properties (thermal stress deformation,thermal stress birefringence, low reflected CR, low blue transmitted CR,orientational sensitivity, etc.) which cause the design of a highcontrast modulation optical system to be less than optimal. Certainly,the performance and design of a modulation optical system would benefitby the existence of improved wire grid polarization beamsplitters. Inparticular, if the performance of the wire grid polarizationbeamsplitter 340 was enhanced, relative to thermal sensitivity,contrast, transmission, and wavelength response (particularly in theblue), modulation optical system 300 could be constructed with fewercomponents and higher net transmission.

FIG. 5 illustrates the general concepts for a new double sided wire gridpolarizer 400 according to the present invention. Most simply, doublesided wire grid polarizer 400 consists of a dielectric substrate 405(preferably glass) with a parallel pattern of sub-wavelength wires 430and grooves 440 formed on a first surface 410 and a second parallelpattern of sub-wavelength wires 430 and grooves 442 formed on a secondsurface 420. As with the prior art devices, these sub-wavelength wiresare conductive electrodes, which are likely made of a metal such asaluminum. First surface 410 is nominally parallel to second surface 420,while the pattern of the sub-wavelength wires 430 of the first surface410 and the pattern of sub-wavelength wires 430 of second surface 420are also nominally parallel to each other. However, the wire patterns onthe two surfaces are not necessarily identical, and indeed, preferablyare different in a controlled and deliberate fashion. It should beunderstood sub-wavelength wires depicted in FIG. 5 are greatlyexaggerated in scale, in order to illustrate their general nature.

Certainly, other opportunities have been suggested for improving theperformance of wire grid polarizers generally, and wire grid polarizersfor the visible spectrum in particular. While U.S. Pat. No. 6,108,131describes the basic structure and properties for visible wavelength wiregrid polarizers, U.S. Pat. No. 6,122,103 proposes a variety ofimprovements to the basic wire grid structure. For example, U.S. Pat.No. 6,122,103 provides methods to broaden the wavelength spectrum andimprove the efficiency and transmitted contrast across the wavelengthspectrum of use without requiring finer pitch structures (such as˜λ/10). In particular, a variety of techniques are employed to reducethe effective refractive index (n) in the medium surrounding the wiregrid, in order to shift the longest wavelength resonance band to shorterwavelengths. This is accomplished most simply by coating the glasssubstrate with a dielectric layer which functions as ananti-reflectional (AR) coating, and then fabricating the wire grid ontothis intermediate dielectric layer. More recently, U.S. Pat. No.6,243,199 describes how wire thickness, wire pitch, groove width, andwire shape, can be varied to optimize transmission and contrast of thetransmitted polarized beam.

Alternately, U.S. Pat. No. 6,532,111 (Kurtz et al.) suggests analternate design for a wire grid polarizer in which each wire isfabricated with an intra-wire sub-structure of alternating metal anddielectric layers (see FIG. 8 c). Unlike the improvements suggested byU.S. Pat. Nos. 6,122,103 and 6,243,199 this application describesopportunities to potentially improve both the reflected beam contrastand the transmitted beam contrast, and thereby improve the combinedoverall contrast (Cw). As noted previously, in optimizing the design ofwire grid polarizers generally, and wire grid polarization beamsplittersin particular (as used in a modulation optical system 300), the overallcontrast is limited by the reflected beam contrast.

The double sided wire grid polarizer 400 of FIG. 5 is an improved wiregrid device that not only has the potential to improve the overallpolarization contrast, but also provides reduced sensitivity to thermalloading, particularly when it is used within an appropriate modulationoptical system 300, (see FIG. 6 a). As stated previously, double sidedwire grid polarizer 400 consists of a dielectric substrate 405 with aparallel pattern of sub-wavelength wires 430 and grooves 440 formed on afirst surface 410 and a second parallel pattern of sub-wavelength wires430 and grooves 442 formed on a second surface 420, where thesub-wavelength wires are nominally metal conductive electrodes. Firstsurface 410 is nominally parallel to second surface 420, while thepattern of the sub-wavelength wires 430 of the first surface 410 and thepattern of sub-wavelength wires 430 of second surface 420 are alsonominally parallel to each other. It should be understood that eachsub-wavelength wire (420 and 430) has a length that is generally largerthan the wavelength of visible light (at least >0.7 μm), and that inactuality the wire lengths are several millimeters, or even centimetersin extent.

Double sided wire grid polarization beamsplitter 400 has a firstimportant difference compared to the existing devices with metal wiresdeposited only on one side, which is that both surfaces will experienceheating due to light absorption. As a result, the double sided wire gridpolarization beamsplitter 400 will be heated more uniformly than theequivalent single sided device. Heating of this device under the largeheat (light) loads required for high lumen projection applications cancause the device to deform, thereby affecting the transiting wavefrontsand the projected on screen image quality. This is particularly true ina modulation optical system 300, as shown in FIG. 4, where the imagebearing light beam 390 is reflected of the second surface of the wiregrid polarization beam splitter 340. Alternately (see FIG. 6 b),modulation optical system 300 could be configured for image bearinglight beam 390 to transmit through the wire grid polarization beamsplitter 340 on its way to the projection lens and screen, but thesystem would then suffer the added aberrations from transmission througha tilted plate. Alternately, the substrate 345 can be made from a lowabsorption glass such as fused silica or a low stress birefringent glasssuch as SF-57, but only at significant expense, and without providingthe other advantages of the double sided structure. The typicalconstruction of a wire grid polarizer also includes the use of adielectric stack coating (an AR coating) on the side opposite that ofthe metal wire grid structure. The stress induced by this stack willtypically vary differentially, both nominally and upon heating, ascompared to the residual stress of the metallic grid coating. Thisuneven stress buildup, upon absorption and heating of the substrate willinduce wavefront degradation. By comparison, as the double sided wiregrid polarization beam splitter 400 will experience heating from lightabsorption within the metal wires on both sides, the device will beheated more uniformly, creating a more balanced surface stress, and thuswill deform less, thereby affecting the transiting wavefronts less.Furthermore, this more uniform heating should also induce less thermalstress birefringence within the substrate 405, thus also enhancing theoverall system contrast of the projected image.

The concept of a double sided wire grid polarization beam splitter 400not only offers the potential for reduced thermal sensitivity, but alsoopportunities to design a better wire grid polarization beamsplitter anda better modulation optical system. This can be understood byconsidering the overall contrast (Cs) for modulation optical system 300of FIG. 6 a (as before, ignoring the LCD contribution), which can beapproximated by:1/Cs=1/(C _(T1) *C _(T2))+1/(C _(R2) *C _(T3))  (5)where C_(T1) is the transmitted contrast through the subwavelength wires430 on the first surface 410 of double sided wire grid polarizationbeamsplitter 400, C_(T2) is the transmitted contrast through thesubwavelength wires 430 on the second surface 420 of double sided wiregrid polarization beamsplitter 400, C_(R2) is the reflected contrast offthe subwavelength wires 430 on the second surface 420 of double sidedwire grid polarization beamsplitter 400, and C_(T3) is the transmittedcontrast for the wire grid polarization analyzer 370. As with the priorsystem (FIG. 4), the overall contrast is largely determined by the lowreflected contrast ratio C_(R2) from the wire grid polarizationbeamsplitter. Likewise, as previously, the analyzer contrast C_(T3)needs to be quite high to compensate for the potentially low C_(R2)values. Thus, as before, the specifications for the transmittedcontrasts C_(T1) and C_(T2) could be relaxed, thus potentially allowingthe transmission values T₁ and T₂) to be enhanced for these twosurfaces. Certainly, the functional dependencies of equations (4) and(5) are similar, relative to design adjustments to the constituent wiregrid arrays which could alter contrast and transmission, therebyallowing optimization of system performance. However, the double sidedwire grid polarization beam splitter provides other opportunities toboth optimize device and system performance.

To begin with, pre-polarizer 330 of the prior modulation optical system300 of FIG. 4, effectively becomes a grid of sub-wavelength wires 430 onthe first surface 410 of the double sided wire grid polarizationbeamsplitter 400 of FIG. 6 a. This provides the immediate advantage thatmodulation optical system 300 of FIG. 6 a utilizes one less opticalcomponent, thus potentially enhancing the overall system lightefficiency. Additionally however, use of the double sided structureeffectively positions the “pre-polarizer” wire grid (of surface 410) ata nominal 45° tilt to the incident light beam. As shown in FIG. 7 a,which plots both the transmitted (C_(T1)) and reflected (C_(R1))theoretical contrast (205 and 210) vs. polar angle for the first surfacewire grid (for green light (550 nm), and an exemplary structure(aluminum wires, wire grid pitch (p)=144 nm, duty cycle=45%, wire gridthickness (t)=144 nm)), the transmitted contrast gradually increases vs.polar angle. FIG. 7 b shows the light efficiency in transmission(T_(P1)) and reflection (R_(P1)) for “p” light for this same structurewith 144 nm thick wires. These graphs show that the first surface wiregrid array of a double sided wire grid polarizing beamsplitter 400oriented at 45°, transmits “p” light with a contrast of ˜8,000:1 and atransmission efficiency of ˜87%.

An alternate wire grid structure, identical to the first, but with 100nm thick aluminum wires can be evaluated using FIGS. 7 c and 7 d, whichplot contrast and efficiency vs. polar angle. In this case, thetheoretical transmitted contrast (C_(T1)) for a 45° oriented wire gridarray has fallen to ˜800:1, while the transmission (T_(P1)) is slightlyhigher at ˜91%. As shown in FIG. 7 e, the wire grid array with the 100nm thick wires also provides a nearly even reflected contrast C_(R1) of˜75:1 vs. wavelength, although the transmitted contrast C_(T1) stillshows the blue fall off typical of most visible wire grid polarizers.While this 100 nm exemplary design has not quite provided an even ˜100:1reflected contrast across the entire visible spectrum, the performanceis significantly improved over existing devices (˜10:1 to 40:1 CR, asshown as curve 210 of FIG. 2 a). Yet another alternate wire gridstructure, identical to the first, but with 200 nm thick aluminum wiresinstead of 100 nm or 144 nm thick wires can be evaluated using FIGS. 7 fand 7 g, which plot the theoretical transmitted (C_(T1)) and reflected(C_(R1)) contrast ratios and light efficiencies (T_(P1) and R_(P1)) vs.wavelength. These plots indicate that thicker wires theoreticallyincrease the transmitted “p” contrast significantly (C_(T1)>20,000:1),but at the cost of a significant drop in light efficiency (T_(P1)averaging ˜82%) particularly at short wavelengths.

Considered in their entirety, plots 7 a-g suggest that a thin metal wirestructure is best suited for the first surface 410 of double sided wiregrid polarizer 400, where only a modest contrast (C_(T1)˜100-200:1) isrequired, while higher transmission (T_(P1)) is valued. This suggeststhat an optimal broadband visible spectrum first surface wire grid,using simple wire grids with rectangular wires, may utilize metal wires<100 nm thick, and perhaps only 75-100 nm thick.

As shown in FIG. 6 a, the “p” light transmitted through the wire gridstructure of first surface 410 is further modified (contrast enhanced)by transmission through the second surface 420. Notably, the modeledcontrasts C_(T1) and C_(T2) are essentially identical when the samestructure is applied to both surfaces, although the first surfaceprovides an air to glass transition, while the second surface provides aglass to air transition. In this example, transmitted light beam 150passes through an optional compensator 360 and then encounters spatiallight modulator 310, which is nominally a reflective LCD, which modifiesthe polarization state of the incident light on a pixel to pixel basisaccording to the applied control voltages. Intermediate code values,between white and black, reduce the amount of “On” state and increasethe amount of “Off” state light. The “On” state light, which has beenpolarization rotated, is “s” polarization relative to the wire gridstructure of second surface 420. In modulation optical system 300 ofFIG. 6 a, this “s” state light reflects off double sided wire gridpolarization beamsplitter 400, is transmitted through an optionalcompensator 365 and polarization analyzer 370, and is subsequentlydirected to the screen by a projection lens (neither of which areshown). Considering again equation (5), the overall contrast C_(s)depends on the reflected contrast C_(R2) and the transmitted contrastC_(T3) of polarization analyzer 370. Polarization analyzer 370, which ispreferentially a wire grid polarizer, is oriented so that the “On” statelight, which had “s” polarization relative to the double sided wire gridpolarization beamsplitter 400, sees this same light as “p” state lightrelative to its' own structure. Polarization analyzer 370 thereforeremoves any alternate polarization leakage light accompanying thedesired “On” state beam.

As the final image contrast depends critically on the values C_(R2) andC_(T3), optimization of the second surface wire grid polarizersignificantly impacts the overall result. As shown in FIG. 7 a, thereflected contrast (Rs/Rp) in the green for the 144 nm thick wirestructure peaks (˜2,000:1) near ˜35° tilt, and is still significantlyboosted (˜100:1) at 45° tilt, as compared to normal incidence (˜20:1).This type of broad angular response can be important for fast opticalsystems (f/2.5 for example). Unfortunately, the model of this devicewith 144 nm thick wires shows a very significant variation of reflectedcontrast (Rs/Rp) vs. wavelength, with a dramatic fall-off in the bluespectrum. This compares unfavorably with model for the similar devicewith 100 nm thick wires, which has the relatively even reflectedcontrast response (Rs/Rp) of FIG. 7 e.

In general, reflected contrast (Rs/Rp), unlike transmitted contrast(Tp/Ts), is hard to control and optimize, particularly when thetransmitted light efficiency T_(P) must also be maximized. Certainly,going to significantly shorter pitch (p˜λ/10, for example) would boostthe reflected contrast. Likewise, more complex metal wire structures(tapered wires, rounded wires, stratified wires, etc . . . ; see FIGS. 8a-d) with current wire pitches (p˜λ/4) can be used to various beneficialeffects, as compared to the basic rectangular wires. The modelingsuggests that the wire grid array with basic rectangular profile wirescan be optimized for both contrast (Rs/Rp ˜100:1 average) andtransmission (Tp) by using wires ˜95-115 nm thick. Alternately, thesedevices could be optimized separately for each color channel, R, G, Bfor use in an electronic projection system. In that case, the modelingsuggests that wire thickness used on the second surface 420 of doublesided wire grid polarizing beamsplitter 400 for the green and redchannels may be significantly thicker than would be the case for theblue channel. For example, the device modeled with 144 nm thick wireshas ˜500:1 average reflected contrast Rs/Rp across much of the green andred spectra, while the transmission Tp remains quite high (FIG. 7 d).

If the reflected contrast Rs/Rp could be boosted sufficiently (>500:1,or 2,000:1 for example) for the specified system, then polarizationanalyzer 370 could potentially be eliminated. Alternately, polarizationanalyzer 370 can have an increased polarization extinction to compensatefor the sub-optimal C_(R2) performance of the wire grid on the secondsurface 420 of double sided wire grid polarization beam splitter 400.However, assuming contrast C_(R2) is only moderately high (˜50-100:1) onaverage, polarization analyzer 370 could also be optimized with moderatecontrast (˜100-400:1) and maximized transmission (>90%). Thus, the wiregrid structures provided for first surface 410 and second surface 420 ofdouble sided wire grid polarization beam splitter 400 may not beidentical. Indeed, the wire pitch (p), wire width (w), wire thickness(t) and the wire profile or structure may not be identical from one sideto the other.

The final image quality, relative to contrast, also depends on therejection of the second reflection leakage light. Again considering FIG.6, the “Off” state light returning from the spatial light modulator 310,is transmitted through second surface 420 and first surface 410 ofdouble sided wire grid polarization beam splitter 400. The “Off” statelight at second surface 420 is affected by the wire grid structure onthat surface such that most of this light is transmitted through, whilethe reflected light (Rp) becomes the leakage light which is subsequentlyfurther reduced by polarization analyzer 370. Most of the transmitted“p” polarized light is likewise efficiently transmitted through the wiregrid structure on the first surface 410, and thereby is safely removedfrom the imaging system. There is however a reflection of the firstsurface 410, which is directed towards the imaging system, and whichcould create a defocused ghost image. However, this ghost beam, whichhas a reduced light level compared to the “Off” state leakage lightreflected into the imaging path at second surface 420, is likewiseremoved by polarization analyzer 370.

The double sided wire grid polarization beam splitter of the presentinvention is notably different than the device described in U.S. Pat.No. 4,289,381 (Garvin), which also describes a wire grid polarizer witha wire grid structure utilizing two parallel metal wire grid arrays. Inparticular, the present device is structurally different than the deviceof U.S. Pat. No. 4,289,381, as that prior art device places bothparallel wire grid arrays on a first surface, rather than separately ona first and second surfaces respectively. Thus the prior art U.S. Pat.No. 4,289,381 device doesn't provide the nominally equalized thermalloading of both surfaces as does the device of the present invention.Additionally, U.S. Pat. No. 4,289,381 specifies a device which isstrictly a very high contrast polarizer, in which contrast is maximized(250,000:1) target, while light transmission efficiency may besacrificed. By comparison, the device of the present invention, seeks tooptimize both the contrast and light efficiencies to seek an overalleffect, with high contrast (500:1-4,000:1 CR) and high combinedtransmitted light efficiency (>80%). Finally, the wire grid polarizingdevice of the present invention, with two parallel wire grid structuresresiding on two separated surfaces, is primarily intended for use as awire grid polarization beam splitter, rather than as a normal incidenceoptimized polarizer. Therefore, the device of the present invention,unlike that of U.S. Pat. No. 4,289,381, is designed to optimize both thetransmitted beam contrast (Tp/Ts) and the reflected beam contrast(Rp/Rs), because both contribute to the overall performance of anoverall modulation optical system.

While this double sided wire grid polarization beam splitter has beendescribed both as a discrete device, and as a component within a largermodulation optical system, it should be understood that there arevariations which fall within the scope and context of the presentinvention. For example, the metal wires which comprise the individualwire grid structures can be constructed using a variety of otherstructures beyond the basic rectangular wire profile. For example, asshown in FIG. 8 a, the sub-wavelength wires 530 could have a taperedstructure 532, which slopes into grooves 540. Tapering is an additionaldesign control parameter, similar to selecting the wire pitch (p), thewire to groove (540) duty cycle, and the wire thickness (t), by whichthe contrast, light efficiency, and wavelength performance of a wiregrid array can be optimized. Likewise, as shown in FIG. 8 b, thesub-wavelength wires 530 could have a rounded structure 534. It shouldbe understood that in either case, using tapered or rounded profilewires, wire grid arrays would be formed on both the first surface 510and the second surface 520 of substrate 505, to create a double sidedwire grid polarization beamsplitter 400. The second side wire gridstructures are not shown on FIGS. 8 a-d for simplicity of illustration.

Similarly, FIG. 8 c shows an alternate composite wire grid structure, inwhich the sub-wavelength wires 530 are constructed with intra-wiresubstructures 550 comprising alternating metal layers (560, 562, and564) and dielectric layers (570, 572, and 574). This device employsresonance enhanced tunneling through the metal layers, by means ofinteraction with the intervening dielectric layers, to enhancetransmission of the “p” polarized light and reflection of the “s”polarized light. The design and performance of wire grid polarizersutilizing this type of intra-wire substructure are described in greaterdetail in U.S. Pat. No. 6,532,111, which is incorporated herein byreference.

FIG. 8 d shows yet another alternate wire grid structure in which themetallic sub-wavelength wires 530 are formed on an intermediatedielectric layer 580, which is in turn in contact with substrate 505.The wire structures of FIGS. 8 c and 8 d expand the design parametersmuch further, beyond just control of wire pitch (p), wire width (w),wire thickness (t), wire profile (tapered, rounded, etc . . . ), toenable dramatically enhanced performance. In particular, the reflectedcontrast Rs/Rp, the contrast performance vs. wavelength (particularly inthe blue spectrum) and vs. angle can be enhanced by using these morecomplicated wire structures of FIGS. 8 c and 8 d. In the case of thedouble sided wire grid polarizer, use of these complicated alternatewire structures particularly suggest further opportunities to improvethe reflected contrast C_(R2) for the wire grid structure on secondsurface 420. Likewise, use of these complex structures could allow thewire grid structure on the first surface 410 to be designed to assistthe suppression of the ghost leakage light which returned to the thatsurface from the LCD.

The double sided wire grid polarization beamsplitter of the presentinvention can be fabricated by forming a wire grid array on the firstsurface, flipping the device over, and forming a wire grid array on thesecond surface while protecting the first surface. Alternately, thedevice could be formed by assembling a double sided wire grid polarizingbeamsplitter by combining two single side devices together with anoptical adhesive, liquid, or gel. In some respects, the manufacturingprocess for the double sided structure need not be demanding. Therelative parallelity of the two wire grid arrays, one to the othershould be maintained, with minimal angular misalignment, in order tooptimize transmission. Obviously, in the extreme of mis-alignment, ascrossed polarizers, effectively no light would be transmitted. However,the tolerance of this angular alignment is fairly loose, with ˜0.5°mis-alignment being tolerable. In the case that the two wire grid arraysof the two respective surfaces have the identical pitch (p), it is notnecessary that the two grids are co-aligned with a tightly minimizedgrid offset 445 (see FIG. 5). The sub-wavelength wires, or conductiveelectrodes, are nominally metallic, so as to utilize the very highelectrical conductivity of such materials. It is also beneficial tochose metals that have high optical efficiencies (reflectivities) in thechosen (visible) wavelength band. Therefore, metals such as aluminum,silver, chrome, or nickel, may be used. Presumably conductive electrodescould be formed of other materials, such as ITO (indium tin oxide), butonly if both the electrical conductivity and the optical efficiency werehigh enough to provide the desired effects.

Preferably, the thickness (d) of substrate 405 needs to be sufficient(>200-500 nm depending on the structure) to avoid resonance enhancedtunneling effects between the two grid arrays. Otherwise, with resonanceenhanced tunneling, the two wire grid arrays would effectively act asone device, rather than two parallel devices, and the performance wouldbe greatly changed. In practice, the substrate thickness will likely bein the 0.5-3.0 mm range. Alternately, the double sided wire gridpolarizer could be constructed with a thin (<0.15 mm) substrate, withthe wire grids of the two surfaces deliberately co-aligned, in anattempt to deliberately utilize resonance enhanced tunneling through themetal wires (as in FIG. 8 c), according to the concepts described inU.S. Pat. No. 6,532,111. While such a device could still benefit by theexpected more uniform heating of both sides, as the substrate thicknesswould be minimal, device fabrication would likely be extremelydifficult.

It should also be understood that the double sided wire gridpolarization beamsplitter of the present invention could be utilizedwith the imaging light being transmitted through the device, as shown inFIG. 6 b, rather than reflected off of it as depicted in FIG. 6 a.Likewise, it should be understood that the double sided wire gridpolarization beamsplitter could be used with a pre-polarizer instead ofan analyzer, or by itself, without either pre-polarizer or polarizationanalyzer.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

-   100. Wire grid polarizer-   110. Parallel conductive electrodes-   120. Dielectric substrate-   130. Beam of light-   132. Light source-   140. Reflected light beam-   150. Transmitted light beam-   200. Transmission efficiency curve-   205. Transmitted contrast ratio curve-   210. Reflected contrast ratio curve-   220. Transmission efficiency curve-   225. Reflected contrast ratio curve-   250. Transmitted beam contrast-   255. Reflected beam contrast-   275. Overall contrast ratio-   300. Modulation optical system-   310. Spatial light modulator-   315. Modulator assembly-   320. Illumination beam-   325. Condensor-   330. Pre-polarizer-   340. Wire grid polarization beamsplitter-   345. Substrate-   350. Sub-wavelength wires-   360. Compensator-   365. Compensator-   370. Polarization analyzer-   375. Optical axis-   380. Recombination prism-   385. Projection lens system-   390. Image bearing light beam-   400. Double sided wire grid polarizer-   405. Substrate-   410. First surface-   420. Second surface-   430. Sub-wavelength wires-   440. Grooves-   442. Grooves-   445. Offset-   505. Substrate-   510. First surface-   520. Second surface-   530. Sub-wavelength wires-   532. Tapered structure-   534. Rounded structure-   540. Grooves-   550. Intra-wire substructure-   560. Metal layer-   562. Metal layer-   564. Metal layer-   570. Dielectric layer-   572. Dielectric layer-   574. Dielectric layer-   580. Intermediate dielectric layer

1. A wire grid polarizer for polarizing an incident light beam,comprising: a substrate having a first surface and a second surface; afirst array of parallel, elongated electrically conductive wiresdisposed on said first surface, wherein each of said wires are spacedapart at a grid period less than a wavelength of said incident light; asecond array of parallel, elongated electrically conductive wiresdisposed on said second surface, and wherein said second array of wiresare oriented parallel to said first array of wires; and wherein each ofsaid wires are spaced apart at a grid period less than a wavelength ofsaid incident light.
 2. A wire grid polarization beamsplitter forpolarizing an incident light beam, comprising: a substrate having afirst surface and a second surface; a first array of parallel, elongatedwires disposed on said first surface, wherein each of said wires arespaced apart at a grid period less than a wavelength of said incidentlight; a second array of parallel, elongated wires disposed on saidsecond surface, and wherein said second array of wires are orientedparallel to said first array of wires; wherein each of said wires ofsaid second array are spaced apart at a grid period less than awavelength of said incident light; wherein said wires of both said firstand second arrays have high electrical conductivity; and wherein designsof first and second arrays are optimized differently to providedifferent polarization based responses relative to performanceattributes, including contrast and light efficiency.
 3. A wire gridpolarization beamsplitter for polarizing an incident light beam as inclaim 2 which has a design for said first and second arrays that isoptimized for a portion of the light spectrum, including one or morecolors of red, green, or blue, respectively.
 4. A modulation opticalsystem for providing high contrast modulation of an incident light beamcomprising: (a) a prepolarizer for pre-polarizing said beam of light toprovide a polarized beam of light; (b) a wire grid polarizationbeamsplitter for receiving said polarized beam of light, fortransmitting said polarized beam of light having a first polarization,and for reflecting said polarized beam of light having a secondpolarization orthogonal to said first polarization wherein one of saidpolarized beams of light is then incident onto a reflective spatiallight modulator; (c) wherein said reflective spatial light modulatorreceives said polarized beam of light, having either a firstpolarization or a second polarization, and selectively modulates saidpolarized beam of light to encode data thereon, providing both modulatedlight and unmodulated light which differ in polarization; (d) whereinsaid reflective spatial light modulator reflects back both saidmodulated light and said unmodulated light to said wire gridpolarization beamsplitter; (e) wherein said wire grid polarizationbeamsplitter separates said modulated light from said unmodulated light;(f) a polarization analyzer receives said modulated light, and whichfurther removes any residual unmodulated light from said modulatedlight; and (g) wherein said wire grid polarization beamsplitter is adouble sided wire grid polarization beamsplitter comprising a substratehaving both a first surface and a second surface; (i) a first array ofparallel, elongated wires are disposed on said first surface; (ii) asecond array of parallel, elongated wires are disposed on said secondsurface, where said second array of wires are oriented parallel to saidfirst array of wires; (iii) wherein each of said wires of said arrayshave a high electrical conductivity and are spaced apart at a gridperiod less than a wavelength of said incident light; and (iv) whereindesigns of first and second arrays are optimized differently to providedifferent polarization based responses relative to performanceattributes, including contrast and light efficiency.
 5. A modulationoptical system as in claim 4 wherein said reflective spatial lightmodulator receives said polarized beam of light having a firstpolarization state transmitted through said wire grid polarizationbeamsplitter.
 6. A modulation optical system as in claim 4 wherein saidreflective spatial light modulator receives said polarized beam of lighthaving a second polarization state reflected from said wire gridpolarization beamsplitter.
 7. A modulation optical system as in claim 4wherein either or both of said pre-polarizer and said polarizationanalyzer are a wire grid polarizer.
 8. A modulation optical system inaccordance with claim 7 wherein a compensator is provided to enhance thepolarization contrast response of the spatial light modulator and/orother polarization components.
 9. A modulation optical system forproviding modulation of an incident light beam, comprising: (a) a doublesided wire grid polarization beam splitter with a substrate having botha first surface and a second surface; (i) a first array of parallel,elongated wires are disposed on said first surface; and (ii) a secondarray of parallel, elongated wires are disposed on said second surfacewhere said second array of wires are oriented parallel to said firstarray of wires; (1) wherein each of said wires of said arrays have ahigh electrical conductivity and are spaced apart at a grid period lessthan a wavelength of said incident light; and (2) wherein the designs offirst and second arrays are optimized differently to provide differentpolarization based responses relative to performance attributes,including contrast and light efficiency; wherein said double sided wiregrid polarization beam splitter transmits a first polarization state ofsaid incident light beam and reflects a second polarization state ofsaid incident light beam wherein said second polarization state isorthogonal to said first polarization state; (c) a reflective spatiallight modulator having a plurality of individual elements which altersaid predetermined polarization state of said transmitted polarizedlight beam to provide said image bearing beam that then reflects back tosaid double sided wire grid polarization beamsplitter, said imagebearing beam then reflecting off of said double sided wire gridpolarization beamsplitter; and (d) a polarization analyzer whichtransmits said image bearing light beam and attenuates unwantedpolarization components accompanying said image bearing light beam. 10.A modulation optical system in accordance with claim 9 wherein saidspatial light modulator is a liquid crystal display (LCD).
 11. Amodulation optical system in accordance with claim 9 wherein said doublesided wire grid polarization beam splitter has a design for said firstand second arrays that is optimized for a portion of the visible colorspectrum.
 12. A modulation optical system in accordance with claim 9wherein said double side wire grid polarization beamsplitter is heatedfrom both sides of said substrate, thereby reducing stress inducedbirefringence within said double side wire grid polarizationbeamsplitter.
 13. A modulation optical system in accordance with claim 9wherein a compensator is provided to enhance the polarization contrastresponse of the spatial light modulator and/or other polarizationcomponents.